JP-A-61-153104 discloses, as a conventional cartridge using hollow fiber membranes, a cartridge having the following structure: a plurality of hollow fiber membranes are accommodated in a cylindrical casing, both ends of each hollow fiber membrane are fixed to the cylindrical casing in the upper and lower adhesion and fixation portions, respectively, and the cylindrical casing is a casing integrally molded so as to extend to the upper and lower ends of the hollow fiber membranes. The hollow portions of a large number of the hollow fiber membranes are open in the upper adhesion and fixation portion of the cartridge but are sealed in the lower adhesion and fixation portion, and an adhesive layer in the lower adhesion and fixation portion has a plurality of through-holes. A gas chamber surrounded by the cylindrical casing is formed under the lower adhesion and fixation portion.
This hollow fiber membrane cartridge is for filtration under external pressure and is used after being set in a filtration column. When materials to be filtered are accumulated on the outer surfaces of the membranes, the filtering capability of the membranes is diminished, and therefore a washing procedure for removing the accumulated materials on the surfaces of the membranes is carried out after filtration for a definite time.
For this washing procedure, a method called gas bubbling is adopted which comprises introducing a gas into the hollow fiber membrane cartridge through its lower portion in a filtration column filled with raw water, and vibrating the hollow fiber membranes in a gas-liquid mixed fluid to peel off the accumulated materials on the surfaces of the membranes.
However, in the hollow fiber membrane cartridge, gas bubbling has insufficient washing effect in some cases because both ends of each hollow fiber membrane are fixed to the cylindrical casing, so that the extension and vibration of the hollow fiber membranes are limited. This phenomenon is remarkable and causes troubles in a long-term filtration operation, particularly when the diameter of the cartridge is large.
On the other hand, JP-A-2000-157846 has proposed a hollow fiber membrane cartridge comprising a cartridge head and a bottom ring to which the end of a bundle of hollow fiber membranes is adhered and fixed, wherein the cartridge head and the bottom ring are not connected and fixed to each other, and a plurality of through-holes for gas introduction are provided in the adhesion and fixation layer of the bottom ring. This hollow fiber membrane cartridge has no cylindrical casing and hence is advantageous in that the extension and vibration of the hollow fiber membranes are not limited during gas bubbling, so that it is easy to peel off suspended materials accumulated on the surfaces of the membranes and to discharge the suspended materials peeled off, from the cartridge.
However, in this hollow fiber membrane cartridge, the feed rate of a gas must be limited in order to prevent the bundle of the hollow fiber membranes from bending due to the lift of the bottom ring during the gas bubbling. On the other hand, when the gas bubbling is repeatedly conducted for a long period of time by the use of a large volume of a gas in order to obtain a sufficient washing effect, the bundle of the hollow fiber membranes is repeatedly bent, so that the hollow fiber membranes are cut in some cases. Therefore, in either case, a long-term filtration operation is difficult to carry out stably.
JP-A-10-137552 discloses a cartridge in which an upper adhesion and fixation layer and a lower adhesion and fixation layer are connected to each other by locating a supporting column in the center of a bundle of membranes. This cartridge makes it possible to avoid the buckling of the bundle of hollow fiber membranes during gas bubbling by the location of the supporting column in the center but involves the following problems: insufficient washing effect is obtained because of the low efficiency of contact between bubbles and the hollow fiber membranes, the cartridge has insufficient resistance to a torsion produced during handling or gas bubbling, and the cartridge is difficult to handle because its upper and/or lower adhesion and fixation portion should be held when the cartridge is carried.
In addition, WO98/28066 discloses a cartridge obtained by surrounding the periphery of a bundle of membranes with a perforated cage-like member. This cartridge is disadvantageous in that since only a small washing effect can be obtained therein because the vibration of the hollow fiber membranes during gas bubbling is limited, a large volume of gas needs to be introduced thereinto in order to obtain a sufficient washing effect.